1. Field of the Invention
The present invention pertains generally to the field of solid-state X-ray imagers and displays, and more particularly is an improved method that structurally alters the optical path to reduce or avoid radiation damage to the semiconductor components used to process the detected X-ray images.
2. Background of the Invention
Since few X-rays with energies exceeding 10 KeV are captured by semiconductor-based image sensor arrays (Si, Ge, etc.), the X-ray energies must be converted into a detectable form. The image sensor arrays are processed on silicon and are only sensitive to light with wavelengths at or near the visible spectrum. Therefore, the arrays require an X-ray-to-visible-light converter in order to detect the X-rays. To this end, X-ray sensitive scintillating materials, such as the Gd2O2S:Tb (GOS or GADOX), CsI(TI) or CdWO4 have been used. These materials greatly enhance the detection efficiency of higher energy X-rays in silicon based sensor arrays through the ability of the scintillating materials to scintillate and emit visible light photons proportional to the X-ray energy. The visible light photons are converted to electrical signals by a silicon based image sensor array, such as a Linear Photodiode Array (PDA). When the image sensor array is read out, the array sequentially produces a stream of electrical video signals from each photo-element with amplitudes proportional to the intensity of the X-ray pattern that impinges on the photo-elements.
However, a problem arises in that the scintillation layer on top of the silicon photo-elements will not absorb the X-ray photons completely. Some portion of the X-ray particles penetrates the scintillation layer and is captured by the image sensor array structure, causing irreversible radiation damage to the image sensor array. Therefore, if the image sensor array used in the X-ray imaging system lies in the X-ray path and is not isolated or protected from X-ray exposure, radiation damage will be inflicted on the silicon image sensor array. As a result, the silicon array used in an X-ray imaging system has a limited useful life time.
FIG. 1 is a simplified electrical block diagram describing the signal processing required for an X-ray detector system. Since the present invention involves only the optical and mechanical structures of such systems, the generalized electrical block diagram shown in FIG. 1 is demonstrative of the signal processing circuitry used in all the systems described herein.
In FIG. 1, the detector is a CMOS device with an image sensor array, a PDA, and the readout control circuits for the array. As is known in the art, the PDA is an array of photodiodes with on-chip control circuits for scanning and reading out video signals. In FIG. 1, the PDA is shown with two rows of photodiodes (PD), Row A and Row B. Row A is a dummy row of dark photodiodes used as a reference to differentially cancel any common mode noise from the active video signal, Row B. Row A is covered with metal to shield the photodiodes from light exposure.
Row B has a light sensing area exposed through a narrow slit in the metal to form a narrow aperture over the length of its read line. When the active photodiodes are exposed to imaging light, each diode collects the photons in the immediate area and converts them to signal charges. The signal charges are stored in the depletion layer capacitance of each individual photodiode. The stored charges are read out during the scanning readout process of the PDA. During one line-scan time, which is known as the integration time, each photodiode goes through an integration process. In integration process, each photodiode is read out and then reset to its initial condition to start collecting photons and converting them to charges for the following line-scan time. Since the readout is sequential, while the PDA is continuously scanning, each photodiode sequentially goes through the photon collection (integration) process during one line-scan time.
The scanning process is initiated by a start pulse, SP. Since the integration time is equal to the line-scan time, the line rate of the video signal is determined by the time required to generate the start pulse, which initiates the scanning of the shift register, SR. As the SR shifts a pulse through its register, two rows of MOS switches, SW, that are in series with the PD are accessed. The pulse from the SR closes two switches. One switch is on the dummy video line, VLD, and the other switch is on the active video line, VLA. As the pulse from the SR accesses the SW, the charges from the accessed PD flows out on to the VLA. The photon converted charges are sent to the signal processing circuit, VP, where the charges are differentially added to the reference charges from the VLD, digitized, and stored for the host computer to perform image processing.
To form a Direct Coupled X-ray Detector, a uniform layer of the scintillating material, SCIN, is deposited directly on the sensing areas of the PDA, or a uniform layer of the scintillating material is placed directly on top of the sensing areas of the PDA. The shaded area with diagonal lines in FIG. 1 shows a SCIN layer that has been deposited over the active PD. SCIN is a uniform coated layer that emits photons when its atoms are excited by the impinging beams of the X-ray. The light energies, proportional to the intensity of the X-ray beams, directly expose the active sensing areas of the image sensors and are processed as describe above.
FIGS. 2a-b show the optical and mechanical components of one of the current art X-ray detector systems commonly used today, a Direct Coupled Detector System. This system is the least complicated in terms of fabrication and applications, and therefore results in lower cost than other systems. The details of the drawing are limited to components relevant to the present invention.
FIG. 2a shows an isometric view of the components: the image sensors (IS), the test specimen (TS), the exposing X-ray beam (XPXB), etc. FIG. 2b is a sectional view. In FIGS. 2a-b, the PDA; the test specimen under X-ray imaging (TS); the exposing X-ray beam (XPXB); and the scintillation coating (SCIN) on the sensor die are depicted to show the geometrical relationship among the components involved in X-ray testing of the test specimen, TS.
In operation, the X-ray source emanates the exposing X-ray beam and exposes the test specimen. The X-ray flux patterns are modulated by the specimen under test as the flux pattern passes onto the surface of the scintillation coating. Since the scintillation coating is coated directly onto the surface of the image sensor, the converted light energies proportional to the X-ray flux patterns are integrated by the image sensor array as it generates the image video signals.
The Direct Coupled Detector System in FIG. 2 shows that the exposing X-ray beam passes through the test specimen, the scintillating layer, and the image sensor array. Accordingly the image sensor array receives that portion of the X-ray flux which is not absorbed by the scintillation layer, causing radiation damage on the silicon sensor. In many applications this radiation exposure is intolerable because it drastically reduces the lifetime of the image sensor array, thereby requiring continual replacement and maintenance of the X-ray imaging system.
Although applying the scintillating layer directly to the image sensor is intolerable for many applications, the primary advantages of the method arise from its simplicity in structure and the close proximity of the scintillating layer to the image sensor array, which improves imaging resolution. Among the advantages of this system are that the detectors are simple to fabricate, i.e., the detectors can be fabricated by simply applying a SCIN coating process to existing image array sensors, such as the PDA. This is a great advantage in applications where a shorter lifetime X-ray detector system is required, for example, in destructive testing where the measuring equipment is also destroyed.
Another advantage of a direct coupled detector system arises from the close proximity of the scintillation layer and the photo-element. Since the scintillating coating is in contact with the image plane of the image sensor array, there is little or essentially no space between them. This close proximity gives the detector the ability to retain its optimum resolution and Modulation Transfer Function (MTF).
Another advantage of the system, arising from the close proximity of the PDA and the scintillating layer, is the light coupling efficiency, i.e., there is very little light energy loss in the transmission between the PDA and the scintillating layer. Another advantage, which arises from its simple structure, is that the system can be implemented in a small enclosure. The ability to use the system in a small enclosure also allows the system to be designed as a portable unit.
However, there are also several drawbacks to the Direct Coupled Detector System. The system user must tolerate a shorter lifetime for the X-ray detector system in a given application, and the PDA must be continually replaced. The Direct Coupled Detector System has a high maintenance cost, requires intensive labor, and requires a significant amount of down time.
In addition to the high operating cost of the system, a major disadvantage stems from the properties of semiconductors in general. Not only are image sensors subject to radiation damages, but all semiconductors, to various degrees, are susceptible to damage from X-ray exposure. Some devices are processed for radiation tolerance that provides some degree of protection and increases the life times of the devices for operations under X-ray exposure. This process is very expensive, and can not render the devices completely immune to radiation damage. Accordingly, in an open unprotected X-ray system, such as the Direct Coupled Detector System, all of the semiconductors in the system are susceptible to radiation damage.
Another disadvantage of the Direct Coupled Detector System arises from the noise properties of the PDA. Sensor noise increases with an increasing number of radiation exposures due to the build-up of undesirable charges in the oxide and silicon interface. Therefore, as the system is used, the noise level increases to an intolerable level, and eventually the image sensor (PDA) must be replaced. Since noise build-up is a function of radiation exposure, depending on the specified signal-to-noise ratio in a given system, the noise build-up may be the limiting factor as opposed to overall functional degradation, i.e., the noise build-up may limit the detector system life time more than the overall device functionality.
A third disadvantage of the Direct Coupled Detector System is that the leakage current of the image sensor increases as the interface charge builds up during operation under X-ray exposure. As the leakage current increases, the storage space in the photodiode is decreased until it is rendered unusable. If a large dynamic range is desired in the subject system, the leakage current build-up will limit the usable lifetime of the sensor.
A fourth disadvantage in the current art Direct Coupled Detector System is that when some of the X-ray photons pass through the scintillation layer and are absorbed by the photodiode, large signal spikes are created that increase the noise level of the video signal.
A fifth disadvantage of the Direct Coupled Detector System is that the scintillating materials commonly used, Gd2O2S:Tb (GOS or GADOX), CsI(TI) and CdWO4, are not easily interchangeable if users want to swap among them in order to detect X-rays with different energy ranges.
A second prior art system, the Fiber Optics Coupled Detector System, employs a fiber optics bundle to transmit the light from the scintillating layer to the PDA. The object of this system is to isolate the PDA and its electronic components from the exposing X-ray beam. FIGS. 3a-b summarize the optical-mechanical configuration of a Fiber Optics Coupled Detector System. The components of the system are an X-ray source (XS); an exposing X-ray beam (XPXB); a lead shield (LS) with the slit to form an aperture (AP); a test specimen (TS) undergoing X-ray examination; scintillating layer (SCIN) that is coated onto the surface of an optical flat transparent transmission plate (OTP); an image sensor DIP package (PDA); a fiber optic bond (FB); a fiber optic bundle (OF); and an image sensor (IS). The X-ray-to-light converter assembly (ASY) represents the assembly of the scintillating layer, the optical transmission plate, and the fiber optic bond.
The X-ray source passes through the aperture, the slit in the lead shield, to limit the area of the X-ray beam exposure to the neighborhood of the X-ray-to-light converter assembly. The X-ray-to-light converter assembly converts the modulated X-ray flux densities, proportional to the density patterns in the test specimen, to proportional light intensities. The light intensities are coupled into the fiber optic bundle through the fiber bond. The fiber optic bundle couples the light flux down to and through a second fiber bond that couples the light flux onto the surface of the image sensor, where the light flux is integrated and processed. The fiber optic transmission line gives the detector the ability to remotely place the X-ray-to-light converter assembly, hence isolating the PDA and its associated electronic circuits from the path of the exposing X-ray beam, thereby giving the X-ray detector system the advantage of protection of the radiation sensitive components. Remotely locating the X-ray detector assembly from the electronic assembly separates the optical path from the X-ray path and achieves the objective of protecting the electronic circuit components from radiation damage.
A second advantage of the Fiber Optics Coupled Detector System is the preservation of the resolution. A fiber optic bundle has a relatively high optical resolution. However, the fiber optic bundle does create a disadvantage for the system in that glass fiber bundles are expensive and difficult to fabricate.
A second disadvantage of the fiber optics system is the difficulty of assembly. The glass bundles are difficult to mount and bond. They must be critically aligned and bonded to their transmitting and receiving components to avoid undue optical transmission losses. The alignment constraint is even greater in the case of bonding the fiber optic ends to the surface of the elements of an image sensor because the fiber ends must be cut to exactly match the surface of the image array elements.
A third disadvantage of the Fiber Optics Coupled Detector System is the constraints imposed on the design of the enclosure. The complicated methods used in bonding and mounting the fiber optic bundle require supporting structures within the enclosure. The supporting structure, which needs to be flexible enough to make initial adjustments, must also serve as a rigid mount to ensure that the bonded ends remain stationary in transportation and operation. Especially critical in adjusting and mounting are the contacts between the scintillating layer and the optical flat transparent transmission plate and at the fiber bond on the image sensor surfaces. The degree of careful handling required becomes even greater in a two-dimensional application of this X-ray system.
A fourth disadvantage of the Fiber Optics Coupled Detector System is that the scintillating materials of Gd2O2S:Tb (GOS or GADOX), CsI(TI) or CdWO4 are not easily interchangeable if users want to swap among them in order to detect X-rays with a different energy range.
Accordingly, it is an object of the present invention to provide an X-ray detector system that is long life, compact, and low cost, and that has a simple mechanical structure that lends itself to simple production assembly with minimal requirements for alignment, adjustment and calibration testing.
Another object of the present invention is to reduce X-ray exposure on components which are sensitive to radiation damage by completely isolating or shielding the components in the detector system from X-ray exposure.